This invention relates to an energy conversion system for generating electricity and to methods of controlling such systems, and more particularly, to a variable speed generation (VSG) system and control method utilizing a doubly fed generator.
A VSG system converts available energy from a resource, such as steam, hydro or wind, into variable speed rotational mechanical energy with a turbine. The mechanical energy is converted into electrical energy by a doubly fed generator. The electrical energy is supplied to a load, such as an electric power grid operating at a system frequency. A doubly fed generator is an electrical machine having a rotor with polyphase excitation rotor windings and a stator with polyphase power stator windings. This is in contrast with the synchronous generator, also having a stator with polyphase power windings, but with a rotor having direct current (DC) excitation, as used in conventional generating stations.
As worldwide supplies of our fossil energy resources, such as oil and coal, begin to dwindle or become more expensive to recover, alternative energy resources, for example, wind, solar and hydro, have been turned to for supplying the increasing demand for electric power. However, these alternative energy sources are often erratic in supplying the energy required by the turbine to produce the rotational mechanical energy. Thus far, to the best of the inventors' knowledge, commercially available generation systems have been unable to attain maximum efficiency of the overall energy conversion process under such varying resource conditions, especially because synchronous generator operation is restricted to a specific synchronous speed.
A doubly fed generator requires an energy converter to supply the polyphase excitation power to the rotor windings. Previously available converters have been costly and inefficient, making commercial implementation of a VSG system having a doubly fed generator impractical. For example, the energy converters of previous systems produce excessive detrimental harmonics. To alleviate this, bulky and expensive harmonic filters are required.
Other variable speed generation systems have proposed using an induction or a synchronous generator with an energy converter connected between the generator stator terminals and power grid to supply the power at the system frequency. However, the converter of these configurations had to be large enough to handle the full power and current supplied by the generator or additional transformers were required. The costs of these additional components also detracts from the viability of these known variable speed generation systems as compared to fixed speed generation systems. Thus, the efficient use of the alternative energy resources has been considered to be too costly as compared to the steady energy resources, such as coal and oil.
Other VSG control systems using a doubly fed generator have concentrated on maximizing turbine efficiency and ignored maximizing the efficiency of the total VSG system. For example, if rotor speed were controlled to gain turbine efficiency in relation to the varying conditions of the resource, the doubly fed generator losses may increase. A synchronous generator would be incompatible in such a system, since it is restricted to operation at a synchronous speed. Also, rotor speed control systems ignore changing electrical load conditions of the grid which may detract from the overall efficiency of the VSG system.
Moreover, other contemplated VSG systems using a doubly fed generator have not been able to supply a wide margin of rotor speed variation without causing excessive excitation requirements. To implement these systems, large converters with a high rating were proposed, and resulted in increasing the electrical losses of the VSG system. Alternatively, the margin of rotor speed variation was limited to the point where the VSG system no longer adequately used the mechanical energy supplied by the alternate resource.
Other systems may have ignored attempting to maximize the efficiency of the VSG system because instability problems could arise under certain conditions. Such instability could occur when a system controller changing the rotor speed to maximize the system efficiency fails to supply sufficient excitation current to the generator. Previous systems have contemplated reacting to these instabilities by activating a command to drop the system off the line, that is, to disconnect the generator from the power grid.
Thus, the previously contemplated variable speed generation systems were not economically nor functionally competitive with other fixed speed generation systems.
Due to the expensive nature of the previously contemplated VSG doubly fed generator systems, and the lack of a viable control strategy, the inherent benefits and qualities of the doubly fed generator were not exploited. For example, a doubly fed generator is capable of a net reactive power control over the entire speed range of the generator. Reactive power relates to the leading or lagging of the waveforms of the polyphase AC (alternating current) voltage and current with respect to one another.
The conventional synchronous generator is limited to supplying electrical energy or power to the grid while rotating at a specific synchronous speed. During start-up, the turbine-generator system must be gradually brought up from essentially zero speed to the synchronous speed before the synchronous generator is synchronized or brought on line by closing power brakers which connect the generator to the grid. Synchronization of the conventional synchronous generator with the grid is a critical task requiring human supervision. At synchronous speed, if the generator is engaged out of phase with the grid, the turbine and generator may sustain damage. If synchronization fails without damage to the turbine-generator unit, then, for example in a hydro turbine application, resynchronization requires reclosure of the gates to begin a new synchronization attempt. Attempts to automate the synchronization, to the best of the inventor's knowledge, have thus far been unsatisfactory.
Thus, due to the economic drawbacks of the previously proposed VSG systems using doubly fed generators, one particular advantage of a doubly fed generator system has not been realized. The doubly fed generator is capable of being synchronized to the power grid at essentially zero speed. Thus, if the doubly fed generator were synchronized out of phase, the turbine and generator at essentially zero speed would not suffer the mechanical shock and resulting damage that would be encountered during a full speed out of phase synchronization. Moreover, since such synchronization can occur, in the hydro turbine example, without opening the turbine gates, resynchronization after a failed attempt is not a burdensome time consuming process.
Zero speed synchronization of the doubly fed generator could be accomplished in remote locations by automated means well in advance of supplying the resource mechanical power to the turbine. As explained above, zero speed synchronization of a synchronous generator is not possible, since the synchronous generator is limited to operation at synchronous speed.
Thus, a need exists for a VSG system and method of control that is economically viable in terms of equipment costs and maximum utilization of the alternative energy resources, while delivering electrical power at a maximum efficiency and exploiting the attractive features of the doubly fed generator.